Modification of the impact resistance of SAN resins by the addition of nitrile rubber is common practice. The softening point of the blends can be increased by curing the rubber component to a soft vulcanized state, U.S. Pat. No. 2,439,202. If the rubber is vulcanized to a higher state of cure, the composition becomes a thermoset no longer processable as a thermoplastic. It is known that surface roughness of SAN resin-nitrile rubber blends could be overcome by building up the toluene-insoluble gel content of the nitrile rubber to at least 30% and less than 50% prior to incorporation in the SAN resin, U.S. Pat. No. 2,550,139. Also known are blends exhibiting improved processing characteristics comprising 50% or more SAN resin and a mixture of rubbers consisting of saturated acrylate rubber and nitrile rubber with a gel content within the range of 50% to 80%, U.S. Pat. No. 2,698,313. Curing of such three component blends improves the impact strength of the composition but gives thermosets no longer processable as thermoplastics. The prior art compositions containing nitrile rubber of high gel content are relatively inextensible which makes them unsuitable for applications where parts are subject to severe deformation.